Demonstration of ACROSS sources phased array in a deep underground site

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A hardware system of new seismic active monitoring method, ACROSS, has been developed. The core hardware is a rotational source. The source rotates an eccentric mass with a constant frequency, the rotation generates centrifugal force and the centrifugal force emits harmonic elastic waves in the ground. The emitted waves are extremely coherent and stable in time. Therefore, one is able to observe some temporal variation of the waves that propagates through a fault zone. The results of this research are as follows ;1)ACROSS system was established in our experimental site. An ACROSS source and three bore-hole type accelerometers were settled on a hard rock in a deep underground. In order to synchronize between the rotation of the source and sampling timing, the source and observation system are linked by means of optical cables. Timing signals generated by the source were transported to the AD converter in the observation system.2)Micro-tremors were observed, and the noise condition was evaluated quantitatively.3)The ACROSS source was driven several times. The emitted waves were observed by using three accelerometers that were equipped at 1 m, 300 m and 1000 m far from the source. In a row time series data, one can not detect the emitted harmonic waves. However, because of the coherency of the emitted waves and the synchronization between the source and the receivers, the emitted waves were accurately estimated. From this experiment, we found that the generated harmonic waves were fairly stable in time. The temporal stability of the waves observed at 1 m site was 0.1% in amplitude and 100 micro-seconds in phase.4)Fifteen components of frequency response were observed and sompi-cepstrum method wad applied to the observed data. An arrival time of P wave was detected.